1. Field of the Invention
This invention relates generally to a plasma remelting furnace and more particularly to a fine-grain producing apparatus wherein raw material, such as metal, is turned into a molten pool as a result of heating by a plasma arc and fine grains are produced efficiently from the molten pool and are collected without loss.
2. Description of the Prior Art
In known find-grain-producing apparatus of this type, metallic raw material supplied to the bottom portion of a furnace is heated by a plasma arc until molten and is thereafter formed into fine grains. In this process, the heating region for the raw material is generally restricted and spatially fixed. Consequently, heat conduction in molten metal or inflow of unmolten row material into the heating region accompanying outflow of molten raw material from there must take place in order to melt all the supplied raw material completely. As the result of this, the level of the yield of fine grains is generally low, it takes a considerable length of time to produce a certain amount of fine grains and, to make matters worse, the homogeneity of the obtained fine grains is uncertain. An apparatus is also known wherein the direction of a plasma arc produced by a plasma torch is variably adjusted to a certain degree. In such an apparatus, however, the direction of the plasma arc is adjusted, on rare occasions, by an operator, and a plasma arc is spatially fixed basically in the process of producing fine grains.
In conventional apparatus of this type, metallic fine grains are formed in a restricted region near a molten pool and are removed through a few discrete discharge ports provided at the side wall of the furnace. Since the motion of produced fine grains carried by a flow of neutral gas for plasma medium is complicated and the fine grains are moreover directed towards the side wall of the furnace, the fine grains stray through the furnace until they arrive at the discharge ports. Some of the fine grains are subsequently attached to the side wall of the furnace, causing trouble, such as deterioration of electrical insulation, or are piled up in corners of a furnace, resulting in the further decrease in yield.